Energy drinks are addictive and the inputs are cheap.

I suspect that to some degree individual drinks will be boom -> bust, as newer brands cannibalize the market by having (or advertising) superior energy, more caffeine, better comedowns, etc., and with older drinks having a higher likelihood of bad publicity simply on account of having been around longer. MLMs like Herbalife are also operating in the space (all "loaded tea" shops are MLM), so obviously in cases like that there's a ton of pressure producing early growth.